Abstract

This paper presents the liquefaction potential of densely populated Kolkata city, which is situated on the world's largest delta island with very soft and thick alluvial soil deposits. Due to presence of soft alluvium deposits at shallow depths, soil resistance against liquefaction is expected to be less. Additionally, large thickness of soil layers may amplify the ground shaking resulting in high seismic demand on the soil. Here in this study, variation of factor of safety against liquefaction is evaluated with depth at different locations in Kolkota city. The study founds striking results that the Kolkata city soils are less prone to liquefaction even though there is significant ground amplification due to presence of thick soil deposits.

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1. Introduction

When the seismic wave travels through soil deposit, cyclic shear stresses develop within the soil deposit. If the soil is saturated, the stress may build progressive pore water pressures. The progressive built up of pore pressures may eventually become so large that cohesionless soil loses its shear strength. Liquefaction potential of soil at any location depends on both the cyclic load due to seismic action and the soil resistance to liquefaction (Marcuson, 1978).

The effects of soil liquefaction on the built environment can be extremely damaging. Buildings, whose foundations bear directly on sand, experience a sudden loss of support if the soil underneath it liquefies. This will result in drastic and irregular settlement of the building causing structural damage including cracking of foundations and damage to the building structure itself, or may leave the structure unserviceable afterwards even without any structural damage. The irregular settlement of ground may also break underground utility lines. The upward pressure applied by the movement of liquefied soil through the crust layer can crack weak foundation slabs and enter buildings through service ducts, and may allow water to damage the building contents and electrical services. Bridges and large buildings constructed over pile foundations may also lose support from the adjacent soil due to soil liquefaction resulting buckling, bending, or tilting of the foundation after shaking (Tokimatsu et al., 1998; Bhattacharya, 2007; Jakka, 2013; Jakka et al., 2010). Earth embankments such as flood levees and earth dams may lost stability or collapse if the material comprising the embankment or its foundation liquefies (Kramer, 1996).

Kolkata megacity, the study area and capital of West Bengal, is situated in Bengal basin in the eastern part of India. According to the census report of 2001, the population of Kolkata city is about 13 millions. Originally Kolkata city has grown in north-south direction along the eastern bank of Hoogly River. It spreads from 22°20’N to23°00´N and 88°04´E to 88°33´E. Figure 1 shows the position of Kolkata city in India and important features. Three rivers namely Ganga, Brahmaputra and Barak flow in Bengal basin. Kolkata being very old as well as highly developed, have many bridges, subways, multi storied buildings, huge shopping malls, etc. As Kolkata city is situated on the thick sedimentary soft river deposits, various structures built in the city have high risk of liquefaction induced damages. However, there are not many liquefaction studies on this region. In year 2004, Chakrobortty et al. (2004) estimated liquefaction potential using Artificial Neural Network (ANN) technique and concluded that Kolkata is highly vulnerable to liquefaction induced damages.

Figure 1.

Important features, places and borehole locations of the study region

Here in this current article, we assessed the liquefaction potential of Kolkata city soils through SPT field data and presented the results. Seismo-tectonic features and past seismicity of the region are first discussed. Details about the geotechnical field investigations and procedure adopted for the evaluation of liquefaction potential are then covered. Results from the study and their significance for Kokata city are finally discussed.